Systems to disperse or treat household wastewater, sewage, stormwater and the like (hereinafter referred to as “waste water”) are well known in the art. For example, one conventional system for disposing with or dispersing wastewater comprises a septic tank receiving wastewater from the structure serviced by the treatment system, a distribution box receiving effluent from the septic tank and a subsurface absorption field receiving the effluent distributed by the distribution box. Alternatively, the treatment system may omit the distribution box and simply comprise a septic tank delivering effluent directly to the absorption field or may replace the distribution box with a pump or siphon that doses the absorption field with effluent from the septic tank.
The absorption field may be for example a gravel trench installation comprising an underground layer of crushed stone, gravel, synthetic material and/or other suitable material that creates an underground distribution trench or bed for effluent or water. An upper layer of cover or backfill material is disposed over the trench or bed and extends to finished grade. Alternatively, the absorption field may be a leaching chamber installation comprising an underground pre-fabricated leaching chamber or similar structure that creates an underground passageway for effluent or water. Similarly, a layer of cover or backfill material is disposed over the leaching chamber and extends to finished grade. Inspection ports or observation tubes (collectively referred to herein as “inspection ports”) are sometimes provided in the installations described above to enable the liquid level therein to be monitored. Each inspection port typically comprises a cylindrical tube having a diameter in the range of from about three (3) inches to six (6) inches that extends deep enough into the installation to enable the liquid level therein to be measured. Holes are usually provided in the lower portion of the cylindrical tube to allow liquid accumulating in the absorption field to enter the inspection port so that the liquid level in the inspection port corresponds to the liquid level in the absorption field. A removable cap or plug overlies the cylindrical tube to seal the same when liquid level measurements are not being made. The cap or plug is typically positioned at or above the finished grade so that the liquid level in the installation can be checked without digging. In some instances however, owners of these installations bury the caps or plugs typically for aesthetic reasons. This of course makes checking the liquid levels more difficult and time consuming.
As will be appreciated, being able to monitor the liquid level in septic or stormwater absorption fields is important for a number of reasons. In particular, being able to monitor the liquid level in such an absorption field allows the capacity of the absorption field to be evaluated, allows the owner/operator to detect when the absorption field is reaching its ultimate capacity and allows the owner/operator to be alerted before the absorption field becomes overloaded. Overloading of the absorption field is of particular importance as it may result in wastewater or effluent backing up into the structure it services and/or wastewater or effluent breaking out onto the ground surface. Both of these conditions have serious negative public health and environmental impacts due to the possible presence of bacteria in the wastewater or effluent.
In the past, liquid levels in absorption fields have been measured by inserting a measuring stick or other object into the inspection port, removing the measuring stick from the inspection port and visually examining the measuring stick to determine how much of the measuring stick is wet. As will be appreciated, measuring liquid levels in this manner is time consuming, cumbersome and only provides liquid level measurements at snapshots in time.
Mechanical (i.e. mercury) float switches have also been used to sense liquid levels in absorption fields. Unfortunately, these float switches require large diameter tubes to provide for the minimum cord length required in order for the float to travel up and down. Also if different liquid levels are to be sensed, a separate float switch for each liquid level to be sensed is required. Furthermore, as the float switches are in direct contact with the wastewater or effluent, the float switches are prone to fouling as a result of biological matter buildup on the float switches.
Pressure transducers which sense liquid levels by detecting the change in liquid pressure acting on the pressure transducers have also been considered. As with mechanical float switches, these pressure transducers are also prone to fouling as a result of biological matter buildup on the pressure transducers. To avoid this problem, pressure transducers that detect the change in the air pressure in the underground trench, bed or leaching chamber bed as the liquid level therein changes have been used. Unfortunately these pressure transducers tend to produce unreliable results once air pressures dissipate and equalize.
Use of ultrasound sensors in sewer manholes and storm drains has also been considered. For example, U.S. Pat. No. 7,002,481 to Crane et al. discloses a monitoring system including one or more monitoring devices, positioned in sewer manholes, storm drain manholes or catch basins, etc., and a remote monitoring station that communicates wirelessly with the monitoring devices. Each monitoring device comprises sensors, a two-way telemetry unit, a power supply, a processor and supporting hardware, all located in an enclosed, waterproof housing. Each monitoring device is placed within a manhole cavity to obtain depth (e.g., water level) measurements and report the measurements back to the remote monitoring station, which analyzes the data and responds to alert messages when a dangerous water level is detected. The sample and reporting rates of the monitoring devices, as well as the water level threshold values, may be remotely programmable via commands transmitted from the remote monitoring station. An additional sensor may monitor the manhole cover for security purposes.
In addition to monitoring the liquid level in absorption fields and the like, monitoring the liquid level in groundwater wells and other types of wells is also important to detect situations where well levels rise to the point where the levels may interfere with the operation of sewer disposal systems or where bacteria and/or viruses present in the liquid may reach the groundwater table and cause contamination.
Although liquid level sensors are known, there exists a need for a reliable non-contact, liquid level measurement device suitable for use in installations comprising narrow diameter pipes extending into the liquid whose level is to be monitored. It is therefore an object of the present invention to provide a novel non-contact, liquid level measurement device and to an installation incorporating the same.